Kagan asymmetric sulfur oxidation

An excellent account of the entire field is given in Asymmetric Oxidation Reactions T. Katsuki, Ed., Oxford University Press, Oxford, 2001. Special focus on oxidation of hetero atoms, e.g., sulfur, is devoted to chapter 4 by H.B. Kagan,... 

There are several efficient methods available for the synthesis of homochiral sulfoxides [3], such as asymmetric oxidation, optical resolution (chemical or bio-catalytic) and nucleophilic substitution on chiral sulfinates (the Andersen synthesis). The asymmetric oxidation process, in particular, has received much attention recently. The first practical example of asymmetric oxidation based on a modified Sharpless epoxidation reagent was first reported by Kagan [4] and Modena [5] independently. With further improvement on the oxidant and the chiral ligand, chiral sulfoxides of >95% ee can be routinely prepared by these asymmetric oxidation methods. Nonetheless, of these methods, the Andersen synthesis [6] is still one of the most widely used and reliable synthetic route to homochiral sulfoxides. Clean inversion takes place at the stereogenic sulfur center of the sulfinate in the Andersen synthesis. Therefore, the key advantage of the Andersen approach is that the absolute configuration of the resulting sulfoxide is well defined provided the absolute stereochemistry of the sulfinate is known. 

An enantioselective version of the reaction described in Protocol 4 has been recently investigated. The asymmetric oxidation of the sulfur centre was performed with the Kagan s modified Sharpless system [1.3 equiv of cumene hydroperoxide in the presence of a Ti(OPr )4/diethyl tartrate/H20 catalyst] and afforded the corresponding sulfinyl-substituted complexes with high enantiomeric purity (Scheme 6.8). 

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